This invention relates to an adjusting and centering mechanism for a sliding advance lock of a servo drum brake, which lock is mounted so as to be axially slidable against the prestressing force of a centering spring in a fixed guide bracket, wherein the subject mechanism is comprised of:
An adjusting nut for each advance direction, into which nut a rotationally rigid threaded bolt is screwed which engages the end of the respective brake shoe, and which nut bears a coaxial toothed adjusting wheel, which has unidirectionally acting ratchet teeth.
Two two-armed adjusting levers, each swingably mounted on the guide bracket by means of an associated bolt, and each subjected to the force of a spring, wherein one arm of each adjusting lever has a pawl which engages the ratchet teeth of one of the two toothed adjusting wheels, and the other arm of each adjusting lever has an abutting-region which can abut against a structural element affixed to the subject and adjusting and centering mechanism, whereby when the adjusting and centering mechanism undergoes an axial displacement one of the two adjusting levers undergoes an angular displacement over its associated toothed adjusting wheel such that the wheel is rotated in the direction of unscrewing of the threaded bolt, wherein the engagement of the pawls and the toothed adjusting wheels is configured such that when a pawl is moved as a consequence of an actuation of the brake this pawl engages a steep side of a tooth of the said wheel, and when moved as a consequence of a release of the brake, the pawl engages a shallowly sloped side of a tooth (or teeth) of said wheel.
An adjusting and centering mechanism of this type is disclosed in German OS 40 17 096. In that mechanism, each adjusting lever has a control lever associated with it, with the adjusting lever and control lever being connected hinge-like by a tensile spring. The prestressing force of the tensile spring, which is always opposed to the force of the centering spring, urges the adjusting lever against a guide bracket, whereby the adjusting lever abuts against the guide bracket itself or against a part (boss bushing) on the toothed adjusting wheel, with a predetermined detent play in said abutting serving as the basic clearance.
Because the force of the tensile spring on the adjusting lever acts always in the direction of the abutting of the lever, and thereby against the force of the centering spring, when the adjusting and centering mechanism is displaced axially, the force of the tensile spring must always be overcome by the toothed adjusting wheel or the boss bushing in order to initiate an angular excursion of the adjusting lever for the adjusting process. Another disadvantage of this known adjusting mechanism is that the size of the basic clearance between the detents of the adjusting lever and the associated sliding element is predetermined. Adjustment occurs only if the axial displacement of the subject adjusting and centering mechanism exceeds the basic clearance.
German Patent 2,311,166 discloses an adjusting device which is excursion-dependent, accomplishing the adjustment by means of double levers. Here too the size of the basic clearance is provided by boss elements, and the spring force acting on the double lever opposes the sliding advance movement.
German Patent 3,112,550 discloses an excursion-dependent adjusting mechanism wherein the adjusting levers are disposed at 90.degree. with respect to each other on the guide bracket. When the sliding advance lock undergoes an axial displacement, the levers are carried along by means of a connecting lug which is coupled with the toothed adjusting wheel and engages an opening in the lever with a play which determines the basic clearance.
Other excursion-dependent adjusting mechanisms wherein the adjusting levers are swung via boss elements are described in U.S. Pat. Nos. 3,326,331, 3,712,425, 4,570,506 and 4,919,237 and British Patent 2,161,874.
All of these known adjusting mechanisms have the disadvantage that when the sliding advance lock is displaced axially the advance of the adjusting lever which occurs is necessarily accomplished by the element (boss bushing, toothed adjusting wheel, pin, etc.) against which the adjusting lever abuts under the prestressing force of the associated spring.